1. Field of the Invention
The present invention relates to a method of cleaning a process tube and a wafer boat and more particularly, it relates to a process tube cleaning method capable of more efficiently removing film of the silicon oxide group to make the cleaning rate as higher as possible.
2. Description of the Related Art
Generally, film such as group of the polysilicon (Poly-Si), silicon nitride (SiN) and silicon oxide (SiO.sub.2) is formed on each of semiconductor wafers in the course of manufacturing semiconductor devices, using the pressure-reduced or normal pressure CVD apparatus.
In the course of forming film of the SiO.sub.2 group on each of the semiconductor wafers, for example, the wafer boat is loaded in the process tube of the pressure-reduced CVD apparatus and heated under reduced pressure. SiH.sub.4 and O.sub.2 gases are then introduced into the process tube to form SiO.sub.2 film on each of the silicon wafers. When the wafer boat is loaded and unloaded, the temperature in the process tube is kept substantially same as that in the process tube in which the film forming process is being carried out. When the film forming process is conducted, SiO.sub.2 film adheres to the inner wall of the process tube and quartz-made tools in the tube When the layer of SiO.sub.2 adhering to the inner wall of the process tube develops as the film forming process is repeated, it peels off the inner wall of the process tube, scattering its particles in the process tube. These particles adhere to the wafers and contaminate them, thereby lowering the productivity of wafers. In the conventional case, therefore, the temperature in the process tube is lowered to room temperature at a certain interval and the process tube is then detached from the apparatus along with the quartz-made tools. The adhering SiO.sub.2 film is thus removed from the process tube and the tools by wet washing.
When the wet washing is employed, however, it takes a long time to lower and raise the temperature in the process tube and to detach the process tube from the apparatus. The time period during which the apparatus is left inoperative becomes longer accordingly, thereby remarkably lowering the operation rate of the apparatus.
The process tube and the boat loading section of the CVD apparatus must be kept under reduced pressure. A new CVD apparatus having a load lock system for keeping the process tube and the loading section under this state has been therefore discussed these days of its practical use. When the CVD apparatus is provided with this load lock system, however, it will become difficult that the process tube which must be cleaned is detached from the apparatus itself.
Further, when the size of each wafer to be processed changes from 6 inches to 8 inches in the near future, the process tube will become larger in size and it will become more difficult to detach the process tube from the apparatus itself.
Japanese Patent Disclosures Sho 64-17857 and Hei 1-92385 disclose a method of removing reacted products from the inner wall of the process tube by the so-called dry etching. According to this method, etching gas is introduced into the process tube and plasma is generated in it after the wafer boat is unloaded from the process tube. CF.sub.4, NF.sub.3 or SF.sub.6 gas is used as the etching gas.
When ClF.sub.3 is used as the cleaning gas, film of the polysilicon system and silicon nitride system can be removed relatively clear. It has been believed, however, that this ClF.sub.3 gas is not useful to remove film of the SiO.sub.2 system. Therefore, NF.sub.3 is used instead of the ClF.sub.3 gas to remove film of the silicon oxide system from the inner wall of the process tube.
However, the NF.sub.3 gas cannot be used at high temperature because its reactivity is high. When the film of the silicon oxide system laid on the inner wall of the process tube by the film forming process which has been conducted at a high temperature of about 700.degree. C. is to be removed by the NF.sub.3 gas, therefore, the apparatus must be once cooled to a temperature lower than that (or process temperature) at which the film forming process has been carried out.
It takes a long time to cool the apparatus from the process temperature to the cleaning temperature and heat it from the cleaning temperature to the process temperature. Further, it also takes a long time to finish the cleaning process because etching rate becomes low at low temperature. As the result, the apparatus must be left inoperative for more than 24 hours until the cleaning process is finished, thereby lowering the operation rate of the apparatus to a great extent.